1. Field of the Invention
The present invention relates to a piezoelectric electro-acoustic transducer such as a piezoelectric receiver, a piezoelectric sounder, or a piezoelectric speaker.
2. Background of the Invention
In electronic devices and apparatuses, such as household electronic appliances and portable telephones, electro-acoustic transducers have been widely used as piezoelectric sounders to generate acoustic alarms or operational sounds and piezoelectric receivers.
Generally, such an electro-acoustic transducer includes a piezoelectric plate that is bonded to one side or both sides of a metallic sheet to define a vibration plate, the periphery of the metallic sheet is bonded and fixed to a case, and the opening of the case is closed with a cover.
For such vibration plates as described above, a piezoelectric plate which radially vibrates is constrained by the metallic plate which suffers no area-changes, such that an area bending vibration is generated. Accordingly, the acoustic conversion efficiency is low. It is difficult to provide a vibration plate having a reduced size and also provide a sound pressure characteristic including a low resonance frequency.
The Applicants of the present invention have proposed in Japanese Unexamined Patent Application Publication No. 2002-10393 a piezoelectric vibration plate having a high acoustic conversion efficiency. In the piezoelectric vibration plate, two or three piezoelectric ceramic layers are laminated. Main surface electrodes are provided on the front and back main surfaces of the laminate. An internal electrode is provided between the ceramic layers. A side surface electrode to connect the main surface electrodes to each other is provided on a side surface of the laminate. A side surface electrode to be connected to the internal electrode is provided on another side surface of the laminate. The ceramic layers are polarized in the same thickness direction. The laminate vibrates in an area bending mode by application of an AC signal between the main surface electrodes and the internal electrode to generate a sound.
The piezoelectric vibration plate having the above-described structure is a ceramic lamination structure. The two vibration regions (ceramic layers) sequentially arranged in the thickness direction vibrate in the opposite directions to each other. Accordingly, the displacement is increased as compared to the vibration plate in which the piezoelectric plates are bonded to the metallic sheet. That is, an increased sound pressure is obtained.
The above-described piezoelectric vibration plate, although it has a high acoustic conversion efficiency, has problems in that when the vibration plate is supported in a case, the periphery of the vibration plate must be closely sealed, which increases the resonance frequency. For example, in a case in which two opposite sides of a piezoelectric vibration plate with a size of 10 mm×10 mm are bonded to a case, and the other two sides are elastically sealed in such a manner as to be freely displaceable, the resonance frequency is about 1200 Hz, and the sound pressure is considerably reduced in the vicinity of 300 Hz which is the lower limit of the frequency band of human speech.
In the case of piezoelectric receivers, an electro-acoustic transducer which is capable of producing a wide band speed having a substantially flat sound pressure characteristic in the frequency range of 300 Hz to 3.4 KHz, that is, the frequency band of human speech, has been demanded. However, according to the above-described supporting structure, a substantially flat sound pressure characteristic in a wide band cannot be achieved. The resonance frequency can be reduced by increasing the sizes of the case and the vibration plate. However, the size of the electro-acoustic transducer is increased accordingly.
Japanese Unexamined Patent Application Publication No. 4-132497 discloses a flat speaker in which an electric feeding circuit is formed of conductive paste on the inner surface of a sheet member which has the periphery reinforced and supported by a rigid frame. A piezoelectric ceramic plate or a piezoelectric vibration plate including a metallic sheet having a piezoelectric plate bonded thereto is bonded to the feeding circuit. In this case, a substantially flat frequency characteristic in a wide band is attained.
When a unimorph piezoelectric vibration plate, that is, a metallic plate having a piezoelectric ceramic sheet bonded thereto is used, the vibration plate itself vibrates in a bending vibration mode, and thus, the plate operates as a speaker. On the other hand, when a piezoelectric ceramic plate is bonded directly to the sheet member, the piezoelectric ceramic plate is expanded and contracted in the plane direction. Thus, a desired speaker characteristic cannot be attained in some cases. Moreover, if the sheet member is excessively large as compared to the vibration plate, an effective sound pressure characteristic cannot be attained, and the size of an electro-acoustic transducer is increased.